The present invention relates to video signal encoding apparatus operable to encode for transmission two or more signals received on respective input channels, the apparatus comprising an encoder stage for each channel operable to encode the received video signal according to a predetermined coding scheme, as a specified number of data bits per encoded video frame and to output the signal as a data stream having a bit rate determined by the specified number; target setting means coupled to the encoder stages and arranged to set specified numbers for respective encoder stages per video frame or group of frames, such that the sum of the respective encoder stage output bit rates remains substantially constant; and respective buffer means coupled to receive the said variable bit-rate data stream from the respective encoder stages and arranged to output buffered data signals at respectively specified output bit rates.
The technique of maintaining a constant overall encoder output bit-rate, despite variations in the individual channel bit rates is known as joint bit-rate control and its use for multi-program video signal encoding has been found to be advantageous, particularly for video signals coded according to MPEG standards. A system for joint bit-rate control needs technical measures to deal with two problems, namely the bit allocation and the buffer management. To deal with bit allocation, the bit need of the programs (channel signals) is measured and the available bits are spread accordingly over the programs. In the present case however we are concerned particularly with the problem of buffer management, which itself encompasses several sub-problems. Some of the sub-problems are described in "Constraints on variable bit-rate video for ATM-networks" by Amy Reibman and Barry Haskell; in IEEE Transactions on Circuits and Systems for Video Technology, Vol 2, No. 4 December 1992 pp. 361-372. The Reibman and Haskell paper examines the constraints resulting from encoder and decoder buffering in an asynchronous transfer mode (ATM) network, in particular the additional constraints needed to prevent overflowing or underflowing of the decoder buffer when a variable bit-rate data channel links encoder and decoder. They describe a method in which the number of encoded bits for each video frame and the number of bits transmitted across the variable bit-rate channel are selected jointly; whilst this reduces the available options, it is suggested as a necessity imposed by the differing constraints imposed on the transmitted bit rate by the encoder and particularly the decoder buffer.